It is sometimes necessary or desirable to store fluid under pressure in a storage tank under conditions in which the pressure of the fluid in the tank does not increase substantially as more fluid is supplied to the tank. One example is in a reverse osmosis system which includes a reverse osmosis filter and a storage tank. In such a system, the reverse osmosis filter provides filtered product water to the storage tank.
In order for a reverse osmosis filter to work efficiently, at least a predetermined pressure drop across the filter must be maintained. If the pressure in the storage tank increases significantly as product water is supplied to it, the differential pressure across the filter reduces, and if the differential pressure falls below a predetermined limit, the efficiency of the reverse osmosis filter in producing product water deteriorates.
The purified water storage system disclosed in my U.S. Pat. No. 4,552,656 addresses this problem by providing a flexible container for product water and using pressurized air outside of the flexible container to pressurize the container. In Gossett et al U.S. Pat. No. 3,831,757 a large and small piston are employed with the small piston resisting movement of the large piston. The small piston is urged to oppose the large piston movement by virtue of feedwater under pressure acting against the small piston. However, the pressure of the feedwater may vary thereby providing a less predictable force to resist movement of the large piston. Also, with low line or feedwater pressure, there may be insufficient force to lift the product water out of the unit.